Some random thoughts:
I was born in and spent the first 20 years of my life in SLC. Not only would I recommend looking into any abandoned mines that might be anywhere around the area, but also would keep look out for any possible sink holes or anything like that. Major Reservoirs around the 4 corners states are at all time lows, so land masses will be changing.
The truck was parked, the boots were there, am I correct his pistol is missing with him but his shotgun was on the bed? To me that really feels of him going out at night to check on something, possibly seeing or hearing someone or something on the property. I can see him taking his pistol with him and perhaps having the shotgun where he could get to it if needed, especially if something on the property or something was unsettling him. I can also easily see him just doing a routine check of the property. He seems to be the outdoor type and I can see him spending time sitting outside the trailer at the end of the day also. The boots still don't exactly fit. And it keeps standing out to me that if something happened to him, he would know that his parents would know he only has one pair or boots. I can see that as his way of leaving them a message. And to take that one step further, possibly the boots fully intended to point to a previously shoeless bloodied person he has dealt with earlier. All theory imo, of course.
Another thing bothering me is his parents keep insisting he did NOT go out, he worked and that was his life and joy. But it confuses me as to the need for him to stay anywhere else at anyone elses trailer of he never goes anywhere. Am I missing something there?